The present invention relates to a combustion control system for a spark-ignition internal combustion engine employing a variable piston stroke characteristic mechanism (a variable compression ratio mechanism capable of varying a compression ratio xcex5) and a variable valve operating mechanism capable of varying a valve lift characteristic (a valve lift, a working angle, a phase of a central angle of the working angle) of an intake valve.
A compression ratio often denoted by Greek letter xcex5 (epsilon) is generally defined as a ratio (V1+V2)/V1 of the full volume (V1+V2) existing within the engine cylinder and combustion chamber with the piston at bottom dead center (BDC) to the clearance-space volume (V1) with the piston at top dead center (TDC). On the other hand, an effective compression ratio denoted by xcex5xe2x80x2 is generally defined as a ratio of the effective cylinder volume corresponding to the maximum working medium volume to the effective clearance volume corresponding to the minimum working medium volume. These two compression ratios xcex5 and xcex5xe2x80x2 are thermodynamically distinguished from each other. A compression ratio is one of basic parameters for enhanced thermal efficiency. In fixed compression-ratio spark-ignition internal combustion engines, compression ratio xcex5 is set to as high a ratio as possible, taking into account the frequency of detonation or knock. There are several options to avoid such fixed compression-ratio engines from knocking. One way to avoid a combustion chamber knock condition is to simply retard ignition-spark timing, when combustion knock occurs in the engine. Another way to avoid knocking is to vary intake-valve operating characteristics, thereby varying effective compression ratio xcex5xe2x80x2. Moreover, a variable compression ratio device can be added as an antiknock means. In designing combustion control systems, it is important to balance two contradictory requirements, that is, high thermal efficiency and high-response knock control. As is generally known, there is an increased tendency for combustion knock to occur at severe engine operating conditions, such as at high-temperature high-load operation. Knocking occurs under certain conditions wherein the temperature of intake air introduced into the engine cylinder goes high enough owing to heat of compression and heat exchange between the intake air and the cylinder wall and piston, and the last part of compressed air-fuel mixture or end gas explodes suddenly or self-ignites before flame propagation is completed and thus the combustion velocity increases, thereby creating knocking noise and higher thermal and mechanical stresses on internal combustion engine components. As discussed above, engine knock tends to occur owing to a temperature rise in the compressed air fuel mixture. There is an increased tendency for knocking to occur, in particular when the engine coolant temperature rises during continuous high-load operation. Setting compression ratio xcex5 to a low value contributes to knocking avoidance, but results in reduced thermal efficiency, that is, deteriorated fuel economy. In recent years, fixed spark-ignition compression-ratio engines usually employ a knock sensor or a detonation sensor in order to detect cylinder ignition knock and retard the ignition spark-timing, to avoid engine knock while setting compression ratio xcex5 to a relatively high value. Ignition-timing retardation, by way of which a relatively high knocking control response can be attained, is suitable for knocking avoidance. However, ignition-timing retardation deteriorates fuel economy and results in an exhaust-temperature rise. This deteriorates the durability of an exhaust-system catalytic converter. For knocking avoidance, ignition-timing retardation can be combined with variable phase control. In case of the integrated knocking-avoidance control based on both ignition-timing retardation and variable phase control, under a condition that there is an increased tendency for the engine to knock, effective compression ratio xcex5xe2x80x2 is lowered by retarding the phase of the camshaft (in other words, by retarding the intake valve closure timing often abbreviated to xe2x80x9cIVCxe2x80x9d), and as a result the charging efficiency and the temperature of air-fuel mixture on compression stroke can be decreased, thereby preventing an exhaust-temperature rise. One such knocking-avoidance system has been disclosed in Japanese Patent Provisional Publication No. 8-338295. Japanese Patent Provisional Publication No. 11-36906 has disclosed a knocking-avoidance system capable of variably controlling a working angle of an intake valve. For improved fuel economy, an integrated control based on both the variable working-angle control and the variable phase control has been proposed. A variable piston stroke characteristic mechanism, which is capable of variably adjusting a compression ratio xcex5 by varying at least one of a TDC position and a BDC position, has been disclosed in pages 706-711 of the issue for 1997 of the paper xe2x80x9cMTZ Motortechnische Zeitschrift 58, No. 11xe2x80x9d.
The variable piston stroke characteristic mechanism or the variable compression ratio mechanism is effective to avoid engine knock. However, the variable piston stroke characteristic mechanism is inferior to the ignition-timing control in control response. To enhance the control response, the variable piston stroke characteristic mechanism requires a comparatively large-size actuator having a large torque capacity. In contrast to the above, with compression ratio xcex5 kept constant, it is possible to reduce the pumping loss by lowering effective compression ratio xcex5xe2x80x2 by way of the intake valve closure timing (IVC) control, for improved fuel economy. In general, the ignition-timing control is superior to a so-called valve operating control containing variable intake-valve working-angle control and variable phase control, in control response for knocking avoidance. The valve operating control is superior to the variable piston stroke characteristic control, in knocking-avoidance control response. Under a particular condition where a comparatively high knocking-avoidance control response is required, such as during rapid acceleration, it is desirable to properly combine the IVC control having a relatively high control response with the other control. In the case that high engine output torque is required, the quantity of intake air has to be increased. However, the increased intake-air quantity is not always reconciled to a drop in effective compression ratio xcex5xe2x80x2. The IVC control has to be utilized suitably depending on engine/vehicle operating conditions, from the viewpoint of the correlation of knocking avoidance with improved fuel economy and enhanced driveability.
Accordingly, it is an object of the invention to provide a combustion control system for a spark-ignition internal combustion engine with a variable piston stroke characteristic mechanism and at least one of a variable valve lift and working angle control mechanism and a variable phase control mechanism, which avoids the aforementioned disadvantages.
It is another object of the invention to provide an integrated combustion control system for a spark-ignition internal combustion engine with a variable piston stroke characteristic mechanism and at least one of a variable valve lift and working angle control mechanism and a variable phase control mechanism, which is capable of setting a compression ratio and operating conditions of an intake valve optimally depending upon engine/vehicle operating conditions, and of balancing a plurality of requirements, that is, knocking avoidance, improved fuel economy, and enhanced driveability by way of integrated control based on an operating mode selected from a variable piston-stroke characteristic control mode (or a variable compression-ratio control mode), a variable intake-valve working-angle control mode, and a variable intake-valve phase control mode, when combustion knock occurs in the engine.
In order to accomplish the aforementioned and other objects of the present invention, a combustion control system for a spark-ignition internal combustion engine comprises a variable piston stroke characteristic mechanism that changes a compression ratio of the engine, sensors that detect engine speed and engine load, at least one of a variable lift and working-angle control mechanism that simultaneously continuously changes a valve lift of an intake valve of the engine and a working angle of the intake valve and a variable phase control mechanism that changes an angular phase at a central angle corresponding to a maximum valve lift point of the intake valve, the working angle being defined as an angle between a crank angle at valve open timing of the intake valve and a crank angle at valve closure timing of the intake valve, and a control unit being configured to be electronically connected to the variable piston stroke characteristic mechanism, the sensors, and the at least one of the variable lift and working-angle control mechanism and the variable phase control mechanism, for controlling the variable piston stroke characteristic mechanism, and the at least one of the variable lift and working-angle control mechanism and the variable phase control mechanism, depending on the engine speed and the engine load.
According to another aspect of the invention, an integrated combustion control system for a spark-ignition internal combustion engine comprises variable piston stroke characteristic means for changing a compression ratio of the engine, sensor means for detecting engine speed and engine load, at least one of variable lift and working-angle control means for simultaneously continuously changing a valve lift of an intake valve of the engine and a working angle of the intake valve and variable phase control means for changing an angular phase at a central angle corresponding to a maximum valve lift point of the intake valve, the working angle being defined as an angle between a crank angle at valve open timing of the intake valve and a crank angle at valve closure timing of the intake valve, and control means being configured to be electronically connected to the variable piston stroke characteristic means, the sensor means, and the at least one of the variable lift and working-angle control means and the variable phase control means, for controlling the variable piston stroke characteristic means, and the at least one of the variable lift and working-angle control means and the variable phase control means, depending on the engine speed and the engine load.
According to a still further aspect of the invention, a method for controlling combustion of a spark-ignition internal combustion engine employing a variable piston stroke characteristic mechanism that changes a compression ratio of the engine, a variable lift and working-angle control mechanism that simultaneously continuously changes a valve lift of an intake valve of the engine and a working angle of the intake valve so that the valve lift increases as the working angle increases, and a variable phase control mechanism that changes an angular phase at a central angle corresponding to a maximum valve lift point of the intake valve, the working angle being defined as an angle between a crank angle at valve open timing of the intake valve and a crank angle at valve closure timing of the intake valve, the method comprises detecting an intensity of knocking and generating a signal indicative of the intensity of knocking, detecting engine operating conditions including both engine speed and engine load, retrieving a desired working angle, a desired central angle and a desired compression ratio from a predetermined priority control map showing how the working angle, the central angle, and the compression ratio have to be varied relative to the engine operating conditions, controlling the working angle so that an actual working angle is brought closer to the desired working angle, controlling the central angle so that an actual central angle is brought closer to the desired central angle, controlling the compression ratio so that an actual compression ratio is brought closer to the desired compression ratio, controlling an ignition timing from a time when the signal from the knock sensor exceeds a predetermined slice level by way of trace-knock control that a change in the ignition timing is substantially inversely proportional to a change in the compression ratio during a full throttle condition, during an accelerating condition with a small valve overlap period, or during a part load condition with a small intake-pipe vacuum, controlling the ignition timing from the time when the signal from the knock sensor exceeds the predetermined slice level by way of trace-knock control that a change in the ignition timing is substantially inversely proportional to a change in the central angle during an accelerating condition with a large valve overlap period, and controlling the ignition timing from the time when the signal from the knock sensor exceeds the predetermined slice level by way of trace-knock control that a change in the ignition timing is substantially inversely proportional to a change in the valve closure timing during a part load condition with a large intake-pipe vacuum.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.